Experimental Brain Research

, Volume 157, Issue 4, pp 417–430 | Cite as

The scaling of motor noise with muscle strength and motor unit number in humans

  • Antonia F. de C. HamiltonEmail author
  • Kelvin E. Jones
  • Daniel M. Wolpert
Research Article


Understanding the origin of noise, or variability, in the motor system is an important step towards understanding how accurate movements are performed. Variability of joint torque during voluntary activation is affected by many factors such as the precision of the descending motor commands, the number of muscles that cross the joint, their size and the number of motor units in each. To investigate the relationship between the peripheral factors and motor noise, the maximum voluntary torque produced at a joint and the coefficient of variation of joint torque were recorded from six adult human subjects for four muscle/joint groups in the arm. It was found that the coefficient of variation of torque decreases systematically as the maximum voluntary torque increases. This decreasing coefficient of variation means that a given torque or force can be more accurately generated by a stronger muscle than a weaker muscle. Simulations demonstrated that muscles with different strengths and different numbers of motor units could account for the experimental data. In the simulations, the magnitude of the coefficient of variation of muscle force depended primarily on the number of motor units innervating the muscle, which relates positively to muscle strength. This result can be generalised to the situation where more than one muscle is available to perform a task, and a muscle activation pattern must be selected. The optimal muscle activation pattern required to generate a target torque using a group of muscles, while minimizing the consequences of signal dependent noise, is derived.


Human muscle Motor units Optimal motor control Muscle strength 



This work was supported by grants from the Brain Research Trust, the Wellcome Trust and the Human Frontiers Science Program. We thank James Ingram for assistance with computer programming and Prof. P. Bawa for discussions on motor unit twitch and overall force variability in motor unit output


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Copyright information

© Springer-Verlag 2004

Authors and Affiliations

  • Antonia F. de C. Hamilton
    • 1
    Email author
  • Kelvin E. Jones
    • 2
  • Daniel M. Wolpert
    • 3
  1. 1.Institute of Cognitive NeuroscienceLondonUK
  2. 2.Department of Biomedical Engineering, Research Transition FacilityUniversity of AlbertaEdmontonCanada
  3. 3.Sobell Department of Motor Neuroscience and Movement Disorders, Institute of NeurologyUniversity College LondonLondonUK

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